counted_table.lua

local function CountedTable(x)
    assert(type(x) == 'table', 'bad parameter #1: must be table')

    local new_t = {}
    local mt = {}

    -- `all` will represent the number of both
    local all = 0
    for k, v in pairs(x) do
        all = all + 1
    end

    mt.__newindex = function(t, k, v)
        if v == nil then
            if rawget(x, k) ~= nil then
                all = all - 1
            end
        else
            if rawget(x, k) == nil then
                all = all + 1
            end
        end

        rawset(x, k, v)
    end

    mt.__index = function(t, k)
        return rawget(x, k)
    end

    mt.__len = function(t)
        return all
    end

    mt.__pairs = function(t)
        return next, x
    end

    new_t.getcount = function()
        return all
    end

    new_t.pairs = function()
        return pairs(x)
    end

    return setmetatable(new_t, mt)
end

local bar = CountedTable { x = 23, y = 43, z = 334, [true] = true }

local function checkcount(count)
        if _VERSION == 'Lua 5.1' then
                assert(bar.getcount() == count)
        elseif _VERSION == 'Lua 5.3' then
                assert(#bar == count)
        end
end

checkcount(4)
assert(bar.x == 23)
bar.x = nil
checkcount(3)
bar.x = nil
checkcount(3)
bar.x = 24
bar.x = 25
assert(bar.x == 25)
checkcount(4)

if _VERSION == 'Lua 5.3' then
    for k, v in pairs(bar) do
        print(k, v)
    end

elseif _VERSION == 'Lua 5.1' then
    for k, v in bar.pairs() do
        print(k, v)
    end
end